Motion Tracking objects that go out of frame and re-enter later
When an object disappears momentarily form the scene (either it goes out of frame and comes back later, or is occluded by other objects) you can track up to the point where it becomes untrackable or it disappears from the frame. 

When it re-appears track it with new tracker.

Then select both trackers (using Shift select) 

and Join them using Join Track (or press CtrlJ)

 

That way blender will know that is dealing with the same element and not a new one. 

Doing this will allow you to have a more accurate camera/object solve or better motion stabilization, depending on what your goals are for tracking.